The definition generally used for visibility distance, and more particularly for weather visibility distance is as follows: it is the distance beyond which a black object of appropriate size is perceived with contrast that is less than a predetermined value that is of the order of 2% to 5%. Contrast between two objects is commonly defined by Michelson's formula:
  C  =                    L        max            -              L        min                            L        max            +              L        min            
In this formula, Lmax designates the luminance of the lighter object and Lmin the luminance of the darker object.
Other formulae can be used, for example Weber's contrast:
  C  =                    L        max            -              L        min                    L      min      
Calculating visibility distance, and more precisely weather visibility distance, is useful for at least two reasons. Firstly, measuring visibility distance itself can enable the driver of a vehicle to be provided with information enabling driving to be adapted to visibility conditions. Secondly, measuring weather visibility distance does not serve only in a system for detecting objects. More generally, it can be useful in providing driving assistance in any circumstances in which it is necessary to perceive certain components in the surroundings, for example in an application to detecting the road.
Making use of weather visibility distance measurements to provide driving assistance does not only involve adjusting the measurement sensor. Means providing driving assistance based on knowledge about visibility can serve, for example, to warn a driver that operation thereof is degraded, or even that driving assistance is deactivated since conditions lie outside the operating range of said means.